The present invention relates to a disc drive data storage device and, more particularly, to a disc drive having a compliant air bearing slider for proximity recording.
Disc drives of the "Winchester" type are well known in the industry. Such drives use rigid discs coated with a magnetizable medium for storage of digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor which causes the discs to spin and the surfaces of the discs to pass under respective head gimbal assemblies (HGAs). Head gimbal assemblies carry transducers which write information to and read information from the disc surface. An actuator mechanism moves the head gimbal assemblies from track to track across the surfaces of the discs under control of electronic circuitry. The actuator mechanism includes a track accessing arm and a load beam for each head gimbal assembly. The load beam provides a preload force which urges the head gimbal assembly toward the disc surface.
The head gimbal assembly includes a hydrodynamic (e.g. air) bearing slider and a gimbal. The gimbal is positioned between the slider and the load beam to provide a resilient connection that allows the slider to pitch and roll while following the topography of the disc. A conventional catamaran slider includes a pair of raised side rails which face the disc surface and form air bearing surfaces. As the disc rotates, the disc drags air under the slider along the air bearing surfaces in a direction approximately parallel to the tangential velocity of the disc. As the air passes beneath the side rails, skin friction on the air bearing surfaces causes the air pressure between the disc and the air bearing surfaces to increase which creates a hydrodynamic lifting force that causes the slider to lift and fly above the disc surface. The preload force supplied by the load beam counteracts the hydrodynamic lifting force. The preload force and the hydrodynamic lifting force reach an equilibrium based upon the hydrodynamic properties of the slider and the speed of rotation of the disc.
Flying height is viewed as one of the most critical parameters of contact and non-contact recording. As the average flying height of the slider decreases, the transducer achieves greater resolution between the individual data bit locations on the disc. Therefore, it is desirable to have the transducers fly as close to the disc as possible. Flying height is preferably uniform regardless of variable flying conditions, such as tangential velocity variation from inside to outside tracks, lateral slider movement during seek operations and air bearing skew angles.
A catamaran slider develops four pressure peaks at the four corners of the slider. The pressure peaks at the leading edge are produced by a leading edge taper. The pressure peaks at the trailing edge are produced by a low clearance between the side rails and the disc surface. Such a design generates a very stiff air bearing in the pitch and roll directions, and a tight flying height distribution is achieved.
Several enhancements have been made to the traditional catamaran slider. U.S. Pat. No. 5,062,017 discloses a slider having hourglass-shaped air bearing surfaces which are wide at the leading and trailing edges and narrow in the middle. The hourglass shapes move more pressure to the four corners of the slider and provide a more even pressure distribution over various skew angles. The slider therefore has a flying height that is relatively skew insensitive. U.S. Pat. No. 5,287,235 discloses an air bearing slider having truncated side rails and a full length center rail. This slider produces three pressure peaks, two of which are located at the corners of the leading edge. The third is located at the center of the slider along the trailing edge. These sliders are appropriate for traditional contact-start-stop types of interfaces because high pitch and roll stiffness result in a smaller flying height variability.
However, these sliders are not well-suited for recording applications where continuous contact occurs. Flying height has recently been reduced significantly from over 10 microinches to less than one microinch in order to achieve a high aerial density of information on the disc surface. With less than 1 microinch separation between the head and the disc, the recording head and media are in contact with one another even at full disc speed. This operation is referred to as "contact" or "proximity" recording. An advantage of proximity recording is that the head-disc separation is solely determined by the media glide avalanche height. Media glide avalanche height is the average flying height after the slider and the disc asperity have worn in over time. With a very low flying height, an optimal magnetic performance can be achieved. A disadvantage of proximity recording is that continuous contact may produce excessive wear or even catastrophic failure.
With conventional contact-start-stop technology, contact occurs between the head and disc only during startup and shut down of the disc drive. During normal operation, the head and disc are separated by a very thin air bearing, and contact does not occur. In this type of application, a stiff air bearing is preferred, since a stiff air bearing has a flying height that is less sensitive to suspension preload, pitch static angle and roll static angle, and has a tight flying height distribution over the disc surface.
With proximity recording, the contact force should be limited to an acceptable range. Although the contact force approaches zero asymptotically since the slider and disc interface wears in over time, catastrophic failure can be caused by a large initial contact force before the interface has had a chance to wear in.
U.S. Pat. No. 5,473,485 discloses a slider having truncated side rails and a rear center pad. Although this slider is less stiff than a traditional catamaran or hourglass-type slider, it requires a very deep recess, such as greater than 20 microns, to limit slider roll about its center line. With a more shallow recess step, when the slider flies at skew with respect to the disc tangential velocity, air flowing across the recess becomes prepressurized before being transferred to the downstream rail. The upstream rail is pressurized by ambient air. This causes an uneven pressure distribution between the side rails, which in turn causes slider roll.
Although a deep recess reduces slider roll, a deep recess is less desirable to manufacture. Ion milling or other etching processes for such a deep recess are very costly and time consuming. As a result, a deep recess is usually produced by a grinding process in mass production. A typical grinding process for such a slider requires several steps of machining and requires blending or rounding of the rail edges. Variabilities in these fabrication steps make a wide flying height distribution among individual sliders. In addition, design flexibility is greatly constrained by the grinding process.